1. Field
The following description relates to a phase modulation technique, and more particularly, to a phase modulation apparatus and method which reflects a communication mode to apply a variable specification.
2. Description of the Related Art
A variable bandwidth Phase Locked Loop (PLL) is applied to a phase modulation apparatus which is generally used in a communication apparatus. Phase modulation using a variable bandwidth PLL has advantages of low cost, low power consumption, excellent noise characteristics, high modulation precision, etc.
In order to achieve high modulation precision, a PLL has to have a wider frequency bandwidth than that of a modulation signal. This is because a high reference frequency reduces a division ratio N to widen the bandwidth of a loop filter and also shorten a PLL lock time. However, a wide bandwidth of PLL degrades noise characteristics.
In order to solve the problem, a 2-point modulation method has been proposed which sets a modulation bandwidth of a PLL to be narrower than a modulation bandwidth and performs modulation within a PLL band and modulation outside of a variable PLL band at two different points.
According to the 2-point modulation method, since no control signal is transferred to a loop filter when a communication mode is a narrow-band mode, effectively only a 1-point modulation is performed. When the communication mode is a wide-band mode, control signals are transferred to individual communication apparatuses to modulate signals outside of the PLL band.
With development of cognitive access techniques and software defined radios (SDR) terminals, demands for a communication apparatus capable of supporting multiple modes are increasing, however, existing phase modulation techniques have limitations in satisfying these demands.